High Voltage Direct Current (HVDC) transmission uses a voltage source converter, which can independently and rapidly control active power and reactive power, thereby improving system stability, suppressing the fluctuation of system frequency and voltage, and enhancing steady-state performance of a grid-connected AC system. The HVDC transmission has great advantages in the fields of renewable energy grid-connection, distributed generation grid-connection, island power supply, and urban distribution network power supply, etc. As the core device in the HVDC technology, a Modular Multilevel Converter (MMC) is the preferred solution for the current HVDC transmission projects due to its modularization, low switching frequency, and good harmonic performance, etc.
The MMC solution-based HVDC transmission projects which have been put into operation at present adopt a Half Bridge sub-module based Modular Multilevel Converter (HB-MMC) solution. If a short-circuit fault occurs to the DC side of the converter, an AC power supply, an antiparallel diode in the half bridge sub-module, and a short-circuit fault point form a short-circuit loop. Since the high-voltage DC circuit breaker technology and manufacturing process are not yet mature at this stage, it is necessary to isolate the fault circuit by disconnecting the AC circuit breaker, and restarting is made only after the fault current naturally decays to 0. This solution is longer in delay for power restoration, and thus the reliability of power supply is reduced.
To endow a DC fault clearance capability to the converter, domestic and foreign scholars have proposed many novel topologies. The proposer of MMC, German scholar R. Marquart proposes a generalized MMC concept with sub-modules as the basic power units and proposes a novel sub-module topology such as a Full Bridge Sub-Module (FBSM). However, the Full Bridge sub-module based Modular Multilevel Converter (FB-MMC) contains many switching devices and is low in utilization ratio of the switching devices and high in operation loss. With this regard, the patent WO2012103936A1 proposes a Half Bridge and Full Bridge sub-module based hybrid Modular Multilevel Converter (HBFB-MMC) solution, which has the advantages of both HB-MMC and FB-MMC and reduces about ¼ of switching devices compared with the FB-MMC solution while having the DC fault clearance capability, and thus the solution has broad application prospects.
In the HBFB-MMC solution, as shown in FIG. 1, the sub-module based hybrid converter includes at least one phase unit; each phase unit includes an upper bridge arm and a lower bridge arm; the upper bridge arm and the lower bridge arm each include at least one half bridge sub-module, at least one full bridge sub-module, and at least one resistor which are connected in series; and an AC side of the converter is connected to an AC power grid by means of a charging resistor as well as a bypass switch and an incoming switch thereof. The half bridge sub-module includes at least two turn-off devices with antiparallel diodes and an energy storage element. A negative pole of the first turn-off device is connected to a positive pole of the second turn-off device to form a first bridge; a positive pole of the first turn-off device serves as a positive pole of the first bridge; a negative pole of the second turn-off device serves as a negative pole of the first bridge; a connecting point between the first turn-off device and the second turn-off device serves as a first terminal of the half bridge sub-module; the negative pole of the first bridge serves as a second terminal of the half bridge sub-module; the positive pole of the first bridge is connected to a positive pole of the energy storage element, and the negative pole of the first bridge is connected to a negative pole of the energy storage element. The full bridge sub-module includes at least four turn-off devices with antiparallel diodes and an energy storage element. A negative pole of the first turn-off device is connected to a positive pole of the second turn-off device to form a first bridge; a positive pole of the first turn-off device serves as a positive pole of the first bridge; a negative pole of the second turn-off device serves as a negative pole of the first bridge; and a connecting point between the first turn-off device and the second turn-off device serves as a first terminal of the half bridge sub-module. A negative pole of the third turn-off device is connected to a positive pole of the fourth turn-off device to form a second bridge; a positive pole of the third turn-off device serves as a positive pole of the second bridge; a negative pole of the fourth turn-off device serves as a negative pole of the second bridge; a connecting point between the third turn-off device and the fourth turn-off device serves as a second terminal of the full bridge sub-module; the positive pole of the first bridge and the positive pole of the second bridge are connected to the positive pole of the energy storage element, and the negative pole of the first bridge and the negative pole of the second bridge are connected to the negative pole of the energy storage element.
During the uncontrolled charging, all half bridge sub-modules are blocked, and all full bridge sub-modules are blocked. FIG. 5 illustrates a schematic diagram of the uncontrolled charging of the half bridge sub-module. When the current flows in the first terminal, the energy storage element of the half bridge sub-module is connected in series to the charging circuit, and the energy storage element is charged. When the current flows out of the first terminal, the energy storage element of the half bridge sub-module is not connected in series to the charging circuit, and the energy storage element is not charged. FIG. 6 illustrates a schematic diagram of the uncontrolled charging of the full bridge sub-module. When the current flows in the first terminal, the energy storage element of the full bridge sub-module is connected in series to the charging circuit, and the energy storage element is charged. When the current flows out of the first terminal, the energy storage element of the full bridge sub-module is also connected in series to the charging circuit, and the energy storage element is charged. Since the charging duration of the full bridge sub-module is about twice that of the half bridge sub-module, the voltage of the full bridge sub-module is about twice that of the half bridge sub-module during uncontrolled charging. Moreover, in the high-voltage situation, the operations of sub-modules depend on the self-powered supply. In general, the starting voltage of the self-powered supply cannot be low. In this case, the half bridge sub-module is not controlled in the AC uncontrolled charging stage, and the next full-controlled charging process cannot be carried out. Therefore, it is necessary to design a charging method for a sub-module based hybrid converter to raise the voltages of the half bridge sub-modules in the uncontrolled stage of the half bridge sub-modules, thereby increasing the starting point of the sub-module based self-powered supply and reducing the design difficulty of the sub-module based self-powered supply.